The estimation of health hazards due to inhaled atmospheric pollutants depends on knowledge of the amount and location of material retained in the lungs. In order to provide the information on which these estimates can be based, we are proposing an experimental program for the measurement of the distribution and clearance of inhaled radiolabeled aerosols over a wide range of particle sizes and breathing patterns. Three factors have come together to make such an investigation particularly appropriate at this time: 1. The technological advances in recording and analyzing radioactive tracer data that have been made over the past 5 years can greatly improve the quality of the fundamental information to be gained. 2. Considerable experience has been gained in generating controlled aerosols. 3. As the nation faces the possibility of considerable expansion of nuclear and coal-fired power plants, more information is needed in order to judge the relative health hazards of alternative sources of energy.